(1) Field of the Invention
The invention relates to wind turbines and is directed more particularly to a turbine blade assembly in which the turbine blades are reconfigured for maximum performance automatically in the course of operation of the turbine.
(2) Description of the Prior Art
Wind turbines are alternative energy sources with low environmental impact. The basic physical principle of wind turbine operation is to extract energy from the wind environment to rotate a mechanism to convert mechanical energy to electrical energy. In FIG. 1, there is shown a typical horizontal axis wind turbine. The turbine generally includes two or three blades 20 attached to a hub 22. Optimally, the blades 20 are lightweight but very stiff, to resist wind gusts. Many blades employ aerodynamic controls, such as ailerons or wind brakes, to control speed. The hub 22 is connected to a drive train (not shown) and is typically flexible to minimize structural loads. This mechanism is connected to an electrical generator 24. Wind turbines usually employ constant rotational speed generators, though advances are underway to utilize variable speed generators with efficient transformers. Variable speed generators have an advantage in that expensive gearboxes can be reduced or eliminated. The entire mechanism is elevated by a tower structure 26. The higher the tower, the stronger the wind, generally. A control room 28 usually is located near the turbine to monitor wind conditions and employ control strategies on the turbine.
Future applications envision wind turbines connected to a main power grid to provide energy to home and business users. At present, the cost of energy associated with wind turbines is significantly higher than the cost associated with non-renewable energy sources (coal or gas fired turbine generators, for example). The U.S. Department of Energy has a goal of substantially reducing the energy cost for sites where the average annual wind speed is about 15 mph. To do this, turbines must more efficiently generate power at lower wind speeds and must withstand excessive structural loading at high wind speeds. Wind turbines constructed based on current technology shut down at very low (below 6 mph) and very high (above 65 mph) wind speeds. This increases the cost of electricity.
The basis for electrical energy generation resides in the aerodynamics associated with a wind turbine. The turbine generates energy from lift produced on the blades in the presence of wind. FIG. 2A shows the effective lift and drag produced by a turbine blade 20 in operation. The two main sources of velocity the blade 20 xe2x80x9cseesxe2x80x9d are due to the rotation rxcfx89 of the rotor and the oncoming wind Vw. The angle xcex2 is the physical angle of the blade either due to a pitch mechanism or due to the twist along the blade. The angle of attack xcex1 the blade xe2x80x98seesxe2x80x99 is therefore:
xcex1=tanxe2x88x921(Vw/rxcfx89)xe2x88x92xcex2xe2x80x83xe2x80x83(1)
As wind speed increases, the angle of attack xcex1 on the blades increases. The blade pitch and twist is typically designed to optimize the angle of attack near the average wind speed. Thus, at low wind speeds the angle of attack is lower than optimum and the turbine loses efficiency. At very low speeds, there is insufficient energy available to drive the turbine. At high wind speeds the angle of attack of the blade becomes excessively large and can drive the blade into a stall. As a result, the forces and moments on the turbine blades become too high and the turbine is shut down to prevent blade failure caused by excessive dynamic loading.
The above applies specifically to the case in which the wind across the turbine rotor is uniform and perpendicular to the flow. During normal operating conditions, neither assumption is typically valid. The flow across the rotor is usually very non-uniform with horizontal and vertical wind shear components. In addition, much of the time, the flow into the rotor (FIG. 2B, for example) is offset by a certain yaw angle xcex3. Defining the position of the blade in the rotation cycle by xcexa8, there is a normal Vn and a crossflow Vc component of the wind:
Vn=Vw cos xcex3
Vc=xe2x88x92Vw sin xcex3xe2x80x83xe2x80x83(2)
The wind velocity is also modified due to horizontal and vertical wind shear at a given position in the angular rotation cycle:
Vw=Vmean+(r/R)[Vvshear cos xcexa8+Vhshear sin xcexa8]xe2x80x83xe2x80x83(3)
The tangential velocity the blade experiences during the rotation cycle is then:
Vt=rxcfx89+Vc cos xcexa8xe2x80x83xe2x80x83(4)
The instantaneous angle of attach of the blade during the rotation cycle is then:
xcex1=tanxe2x88x921(Vn/Vt)xe2x88x92xcex2xe2x80x83xe2x80x83(5)
During uncontrolled turbine operation, there is significant variation in the local blade angle of attack. For large angle of attack variations, this can result in a phenomena termed xe2x80x9cdynamic stallxe2x80x9d. Experimental field studies have demonstrated that significant dynamic loading can be experienced by the turbine blade resulting in fatigue and potential failure of the wind turbine. This problem is a major cause of increased operational and maintenance costs. An additional consequence is that for high wind speeds, the turbine is rarely operating under optimal conditions in terms of blade angle of attack. For both low and high wind speeds, it is desirable to control the local blade angle of attack to establish optimal operating conditions.
There are essentially two ways to control the blade angle of attack. The first is to vary the rotational velocity of the turbine. This is a major reason that research has been conducted to improve the efficiency of variable speed power transformers. During high wind speeds, it is desirable to increase the rotational velocity of the turbine, and decrease the rotational velocity during low wind speeds. Unfortunately, the efficiency of the transformers are such that it is still more cost effective to sacrifice operating the turbine during high wind states and maintain constant rotational velocity.
Accordingly, there is a need to provide an alternative wind turbine assembly which facilitates control of the angle of attack of the blades, as by actively or dynamically reconfiguring the blades to provide continuous adjustment of the angle of attack, as by local blade pitch angle adjustments and/or by pitching the entire blades.
An object of the invention is, therefore, to provide a wind turbine assembly adapted to twist the turbine blades dynamically to increase efficiency at low wind speeds, and reduce dynamic loads at high wind speeds.
A further object of the invention is to provide a wind turbine assembly having means to control the dynamics of the blade during instances of wind shear and non-zero yaw of the turbine with respect to the wind, such that optimal blade angles of attack can be maintained throughout the entire rotational cycle of the wind turbine.
A still further object of the invention is to provide a wind turbine assembly adapted to effect dynamic blade twist so that wind turbines will start at lower wind speeds, and continue to operate at higher wind speeds.
A still further object of the invention is to provide a wind turbine assembly adapted to adjust the twist of the wind turbine blades so as to increase the lift at low speeds and decrease the lift at high speeds, whereby to increase the range of wind speeds at which wind turbines can practically produce energy, and wherein at any specific wind speed the blades twist is optimized for that speed to improve the overall efficiency of the system.
With the above and other objects in view, a feature of the present invention is the provision of a dynamically reconfigurable wind turbine blade assembly comprising a plurality of reconfigurable twistable blades mounted on a hub, an actuator fixed to each of the blades and adapted to effect the reconfiguration thereof, and an actuator power regulator for regulating electrical power supplied to the actuators. A control computer accepts signals indicative of current wind conditions and blade configuration twist, and sends commands to the actuator power regulator. Sensors measure current wind conditions and current configurations and speed of the blades. An electrical generator supplies electrical power to the assembly. Data from the sensors is fed to the control computer which commands the actuator power regulator to energize the actuators to reconfigure the blades for optimum performance under current wind conditions.
In accordance with a further feature of the invention, there is provided a dynamically reconfigurable wind turbine blade assembly comprising a plurality of blades, each being reconfigurable while in operation to assume a selected configuration, an actuator embedded in each of the blades and adapted to receive electrical power to effect the blade reconfiguration to the selected configuration, and an actuator power regulator for regulating the electrical power supplied to the actuators. A control computer accepts signals indicative of current configuration of the blades, wind speed, rotational speed of the blades, and voltage and current available, and processes the signals, and sends commands to the actuator power regulator, and continuously adjusts the commands in response to the signals received. A blade load sensor is embedded in each of the blades and is adapted to measure deflection of the blade, and thereby the configuration of the blade, and to report to the control computer. Rotational speed sensors are embedded in a hub for the blades and are adapted to measure blade rotational speed and to report to the control computer. A wind speed sensor is disposed proximate a remainder of the assembly, and adapted to measure wind speed and to report to the control computer. An electrical generator supplies electrical power to the control computer and to the actuator power regulator. Data from the sensors is fed to the control computer which commands the actuator power regulator to initiate operation of the actuators to reconfigure the blades to obtain the selected configuration under current wind conditions.
The above and other features of the invention, including various novel details of construction and combinations of parts, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular assembly embodying the invention is shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.